1. Field of the Invention
The present invention relates to an approximately 90K-class (critical temperature Tc=approx. 90 degrees Kelvin) oxide superconducting material and a method of producing the same.
2. Description of the Related Art
The conventional method of raising the critical current density (Jc) of oxide superconducting materials is to establish pinning sites in REBa.sub.2 Cu.sub.3 O.sub.7-x (superconducting phase; abbreviated: 123 phase) by dispersion of about 1-micron particles of RE.sub.2 BaCuO.sub.5 (211) or RE.sub.4 Ba.sub.2 Cu.sub.2 O.sub.10 (422). (RE in the foregoing notations designates one or a combination of two or more rare earth elements including Y.) It is known that 211 can be refined to around 1 micron by addition of Pt or Rh and that 422 can be refined to about the same level by addition of Ce. It is also known that a portion of the added Ce forms fine CeBaO.sub.3 of a particle size of 1 micron or less that disperses into the superconducting phase.
JP-A-(unexamined published Japanese patent application)4-16511 teaches a structure having BaMO.sub.3 (M representing Zr, Sn, Ce or Ti) finely dispersed together with 211 in stacked plate-like 123 phase. Materials having this structure are produced in a temperature gradient. JPA-5-279033, JP-A-5-286719, JP-A-6-1609 teach methods of producing superconductors in which 211 is finely dispersed by adding cerium oxide. They also teach structures having cerium oxide finely dispersed in 123 phase together with 211 and a noble metal such as silver. JP-A-5-58626 describes a superconductor having 211 finely dispersed in 123 phase containing Ce and method of producing the superconductor and teaches that addition of Ce enables fine dispersion of 211 by suppressing its agglomeration and enlargement.
From the viewpoint of achieving high Jc, it is preferable to introduce a large amount of fine non-superconducting particles and other pinning centers in the 123 phase. While 211 and 422 are currently the main non-superconducting phases used to produce pinning sites, the CeBaO.sub.3, SnBaO etc. taught in the foregoing literature also contribute to the pinning site formation, although at a lower rate than 211. The development of still other new substances capable of forming pinning sites is a challenge that demands attention.